rebase -i: interrupt rebase when "commit --amend" failed during "reword"

rebase -i: interrupt rebase when "commit --amend" failed during "reword"

"commit --amend" could fail in cases like the user empties the commit
message, or pre-commit failed.  When it fails, rebase should be
interrupted and alert the user, rather than ignoring the error and
continue on rebasing.  This also gives users a way to gracefully
interrupt a "reword" if they decided they actually want to do an "edit",
or even "rebase --abort".

Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>